Atomistic modelling studies were made of the axial relaxation of a Nb-substituting Fe3+ cation, related to an associated O vacancy. The methods used were shell-model simulations and embedded-cluster calculations. These results showed consistently that the Fe impurity was displaced in the opposite direction to the O vacancy. The present results differed from interpretations which were based upon the superposition model. It was shown that slight modifications of Fe-specific superposition model parameters were sufficient to give agreement between superposition-based interpretations and shell-model or embedded-cluster calculations. The relaxation pattern which was found in this special case was expected to be valid for most axial defect aggregates of this particular type.

H.Donnerberg: Physical Review B, 1994, 50[13], 9053-62